Edexcel IGCSE Physics · Spec 7.1
Atomic Structure & Isotopes
The structure of the atom and what isotopes are.
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Atomic Structure & Isotopes
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Explained
Writing an atom down, and what the two numbers mean
An atom has a tiny nucleus of protons and neutrons, with electrons in shells around it. The nucleus holds nearly all the mass in an extremely small volume, and the electrons occupy nearly all the space.
How small is worth a moment. If the atom were a sports stadium, the nucleus would be a pea at the centre. Everything else is empty, which is exactly what Rutherford's scattering experiment showed when most alpha particles passed straight through a metal foil.
The particles
- Proton: relative mass 1, relative charge plus 1, in the nucleus.
- Neutron: relative mass 1, relative charge 0, in the nucleus.
- Electron: relative mass negligible, relative charge minus 1, in shells around the nucleus.
A neutral atom has as many electrons as protons. Lose or gain an electron and it becomes an ion, but the nucleus is untouched, so it is still the same element.
Nuclide notation
An atom is written with two numbers beside its symbol. The nucleon number, or mass number, goes on top: the number of protons plus neutrons. The proton number, or atomic number, goes below: the number of protons.
Subtract the bottom from the top and you have the number of neutrons. That subtraction is the single most used calculation in the whole of the radioactivity topic.
The proton number decides the element. Carbon always has six protons; anything with six protons is carbon.
Isotopes
Isotopes are atoms of the same element with the same proton number but different numbers of neutrons.
Carbon 12 and carbon 14 both have six protons. One has six neutrons and the other has eight. Chemically they behave identically, because chemistry depends on electrons and the number of electrons has not changed.
What can differ is stability. Carbon 12 is stable and carbon 14 is not, so carbon 14 decays and emits radiation. That is why isotopes matter in physics: a radioactive isotope of an element is chemically indistinguishable from a stable one, which is precisely what makes tracers work.
What the mark scheme accepts and rejects
Edexcel International GCSE Physics mark schemes mark this notation number by number. On a decay equation, separate marks are given for the proton number of the alpha particle being 2, for its nucleon number being 4, and for the nucleon numbers balancing across the equation, with an error carried forward allowed from an incorrect alpha.
Three marks for three numbers means that filling in every box is worth doing even when you are unsure of one, because each is credited independently.
The same care shows up in the vocabulary. Defining half-life, one mark scheme accepts the halving of the activity, of the number of nuclei, or of the mass of the isotope. It allows atoms in place of nuclei, and then instructs the examiner to ignore the mass of the substance or sample.
The distinction there is exact. The mass of the radioactive isotope in a sample does halve. The mass of the sample as a whole does not, because the decayed atoms are still sitting there as a different element. One word changes a correct statement into a wrong one.
Why the numbers are called what they are
Nucleon is simply a word covering protons and neutrons together, since both live in the nucleus. So the nucleon number counts everything in the nucleus, and the proton number counts one kind of thing in it.
You will also see them called mass number and atomic number, which mean exactly the same. Both sets of names appear in past papers, so recognising all four terms is worth a moment now rather than in the exam.
Spec 7.1
What you need to know
- Describe the structure of an atom
- Use atomic number and mass number
- Explain what an isotope is
Active recall
Quick check
Answer each question before opening the answer.
What are the relative charges of a proton, neutron and electron?
Proton +1, neutron 0, electron −1.
What are isotopes?
Atoms of the same element with the same number of protons but different numbers of neutrons.
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